The invention relates generally to semiconductor devices having improved reliability. In particular, the invention relates to reducing the defects in semiconductor materials employed in bipolar semiconductor devices.
Silicon carbide is often employed in various semiconductor applications such as electronic devices, including bipolar devices. Silicon carbide has a wide bandgap, a high breakdown electric field, a high thermal conductivity, and a high-saturated electron drift velocity, which makes it a desirable candidate for use in semiconductor devices. Moreover, it is technologically feasible to grow large crystals of silicon carbide to be used in various electronic applications. Also, silicon carbide is a physically robust material that has a high melting point.
However, because of its physical properties, silicon carbide is also relatively difficult to produce. For example, silicon carbide exists in various crystal structures, also known as polytypes. As will be appreciated, there are as many as 150 polytypes of silicon carbide. These polytypes are formed by different stacking orders of the silicon carbide layers in the crystal structure and are separated by relatively small thermodynamic differences. Therefore, these polytypes may be formed by a slight temperature variation during the manufacturing process. Hence, growing single crystal substrates and high quality epitaxial layers in silicon carbide has been, and remains, a difficult task.
Moreover, it has been observed that the performance of the silicon carbide devices tend to degrade upon prolonged use. In particular, forward voltage under forward bias tends to increase with time in bipolar junction devices. This effect is also known as bipolar degradation. As will be appreciated, the term “bipolar” refers to any device in which operation is achieved at least partially by means of minority carrier injection such that the conduction through some region of the device is accomplished using both electrons and holes as carriers simultaneously, or a device in which, during forward conduction, there is at least one forward biased p-n junction. The increase in forward voltage in bipolar devices is generally attributed to crystal defects such as stacking faults bound by edge dislocations. Under forward bias, the stacking faults tend to progress, in process causing the forward voltage to increase.
Further, leakage current caused by screw dislocations also adds to the performance degradation of the bipolar devices. Screw dislocations are the primary cause of high leakage current. As will be appreciated, high leakage current lowers the signal to noise ratio, thereby limiting the use of silicon carbide devices.
Although a number of advances have been made in the growth of silicon carbide and its use in devices, it is desirable to further minimize the defects in silicon carbide to make it a viable choice for commercial products. Accordingly, there exists a need for a silicon carbide substrate having a relatively low defect density, which can be processed at low cost and employed in a semiconductor device.